Screening of some British simuliids for susceptibility to experimental Onchocerca lienalis infection.

Ten species of Simulium from Britain were tested for susceptibility to Onchocerca lienalis infection by intrathoracic injection of microfilariae recovered from the skin of cattle at slaughter. All of the species examined supported development of a proportion of the microfilariae to third-stage larvae, but this was greatest at 53% in S. ornatum s.l., which is a known natural vector in Britain. Inocula of 20, 30 or 50 microfilariae resulted in similar levels of mortality in S. ornatum s.l. (42-53%), but a rise in the dose above 20 microfilariae per fly increased the proportion which developed to third-stage larvae. In a series of comparative experiments employing S. ornatum s.l. as a reference positive for blackflies of unknown susceptibility to infection, 6 other species (S. reptans, S. erythrocephalum, S. lineatum, S. equinum, S. nitidifrons and S. austeni) allowed a proportion of the microfilariae to reach third-stage larvae equivalent to 37-50% of those which developed in S. ornatum s.l. A small sample of S. angustipes supported development as well as its respective control group of S. ornatum s.l., but S. vernum and S. aureum were relatively refractory and were the only species in which third-stage larvae did not accumulate in the head. Fly survival rates varied between the species, but all were below those in S. ornatum s.l. It is concluded that some of these blackflies offer promise as surrogate vectors of O. lienalis in the laboratory and their possible role in natural transmission should receive further attention.